The invention relates to methods and apparatus, both devices and systems, for control of NOx in catalytic combustion systems, and more particularly the control of NOx produced downstream of the catalytic reaction zone of a combustor, while at the same time maintaining the same power output yet low CO, by reducing combustion residence time, inter alia, through control of the location of the homogeneous combustion wave.
Gas turbines are used for a variety of purposes, among them: motive power; gas compression; and generation of electricity. The use of gas turbines for electrical generation is of particular and growing interest due to a number of factors, among them being modularity of design, generation output capacity to size and weight, portability, scalability, and efficiency. In addition, gas turbines generally use low sulfur hydrocarbon fuels, principally natural gas, which offers the promise of lower sulfur oxides or SOx pollutant output. This is particularly important in urban areas that use, or can use, gas turbines for power generation, as they are attractive for power-grid supply in-fill to cover growing power needs as urban densification occurs.
Gas turbines tend to operate with a high turbine inlet temperature, in the range of from about 1100xc2x0 C. for moderate efficiency turbines, to 1500xc2x0 C. for modern high efficiency engines. To achieve these temperatures at the turbine inlet, the combustion system must produce a somewhat higher temperature, generally 1200 to 1600xc2x0 C. as a result of some air addition due to seal leakage or the purposeful addition of air for cooling of portions of the gas turbine structure. At these temperatures, the combustion system will produce NOx. The amount of NOx produced increases as the temperature increases. However, to meet ever more stringent emissions standards, turbine operating conditions must be controlled so that the amount of NOx produced does not increase.
A typical gas turbine system comprises a compressor upstream of, and feeding compressed air to, a combustor section in which fuel is injected and burned to provide hot gases to the drive turbine located just downstream of the combustor. FIG. 1 shows such a prior art system employing a catalytic combustion system in the combustor section. FIG. 1 shows a conventional system of the type described in U.S. Pat. No. 5,183,401 by Dalla Betta et al., U.S. Pat. No. 5,232,357 by Dalla Betta et al., U.S. Pat. No. 5,250,489 by Dalla Betta et al., U.S. Pat. No. 5,281,128 by Dalla Betta et al., and U.S. Pat. No. 5,425,632 by Tsurumi et al. These types of turbines employ an integrated catalytic combustion system in the combustor section. Note the combustor section comprises the apparatus system between the compressor and the drive turbine.
As shown in FIG. 1 the illustrative combustor section comprises: a housing in which is disposed a preburner; fuel source inlets; catalyst fuel injector and mixer; one or more catalyst sections; and a post catalyst reaction zone. The preburner burns a portion of the total fuel to raise the temperature of the gas mixture entering the catalyst, and some NOx is formed there. Additional fuel is introduced downstream of the preburner and upstream of the catalyst and is mixed with the process air by an injector mixer to provide a fuel/air mixture (F/A mixture). The F/A mixture is introduced into the catalyst where a portion of the F/A mixture is oxidized by the catalyst, further raising the temperature. This partially combusted F/A mixture then flows into the post catalyst reaction zone wherein auto-ignition takes place a spaced distance downstream of the outlet end of the catalyst module. The remaining unburned F/A mixture combusts in what is called the homogeneous combustion (HC) zone (within the post catalyst reaction zone), raising the process gases to the temperature required to efficiently operate the turbine. Note that in this catalytic combustion technology, only a portion of the fuel is combusted within the catalyst module and a significant portion of the fuel is combusted downstream of the catalyst in the HC zone.
Each type of drive turbine has a designed inlet temperature, called the design temperature. For proper operation of a gas turbine at high efficiency, the system or operator must control the outlet temperature of the combustor section to keep the temperature at the design-temperature of the drive turbine. This can be a very high temperature, in the range of 1100xc2x0 C. for moderate efficiency gas turbines and as high as 1400 to 1600xc2x0 C. for modern high efficiency engines. As shown in FIG. 1, at these high temperatures, NOx forms in the xe2x80x9cPost catalyst reaction zonexe2x80x9d of the combustor section. Although the NOx level produced in the post catalytic combustion zone is typically low for natural gas and similar fuels, it is still desirable to reduce this level even further to meet increasingly stringent emissions requirements.
FIG. 2 shows the relationship between the temperature in the post catalyst reaction zone and the amount of NOx produced, for a catalytic combustion system of the type shown in FIG. 1. At temperatures below about 1450xc2x0 C., identified in the figure as Region A, the level of NOx produced is below 1 ppm. As seen in FIG. 2, at temperatures above about 1450xc2x0 C., the Region B lower boundary, the NOx level rises rapidly, with 5 ppm produced at 1550xc2x0 C., and even higher levels above that temperature, on the order of 9-10 ppm or higher.
The formation of NOx at a high temperature is a kinetically controlled process. A portion of the NOx, called xe2x80x9cPrompt NOx,xe2x80x9d or xe2x80x9cFennimore NOx,xe2x80x9d forms in the region of the combustor where rapid reactions occur. The amount of Prompt NOx formed depends on the fuel-to-air ratio and final flame temperature, but this Prompt NOx stops forming once the flame-front has consumed most of the fuel. A second pathway to the formation of NOx is the xe2x80x9cThermal NOxxe2x80x9d or xe2x80x9cZeldovich pathway,xe2x80x9d in which NOx is formed continuously at high temperatures and in quantities dependant only on time and temperature. In typical gas turbine systems with residence times in the range of 10 to 20 ms (milliseconds), the prompt and thermal pathways produce roughly the same amount of NOx.
In most combustion processes, reaction of the fuel occurs in a flame that is fixed in location by a flame holder. The flame holder can be either a physical object or an aerodynamic process to anchor or stabilize the flame. Physical elements include bluff bodies, v-gutters, or other such mechanical parts that recirculate the gas stream to stabilize the flame. Aerodynamic stabilizers include physical elements such as swirlers and vanes and such modifications as expanded flow area to stabilize the flame. Flame temperature, temperature profile, physical dimensions of the combustor, and other such features determine the thermal NOx formation. For example, the designer cannot change thermal NOx levels without changing the volume or length of the combustor or the position at which the combustor design anchors the flame.
In the case of a catalytic combustion system using the technology described in the above-identified U S Patents, and other references, only a portion of the fuel is combusted within the catalyst and a significant portion of the fuel is combusted down stream of the catalyst in a post catalyst homogeneous combustion (HC) zone. FIG. 3 schematically illustrates the downstream HC zone.
The top portion of FIG. 3 is an enlarged schematic of a portion of FIG. 1 showing the major components of a catalytic combustion system 12 located downstream of the preburner. The catalytic combustion system includes a catalyst fuel injector 11, one or more catalyst sections 13 and the post catalyst reaction zone 14 in which is located the HC (homogeneous combustion) zone 15. The bottom portion of FIG. 3 illustrates the temperature profile and fuel composition of the combustion gases as they flow through the combustor section described above. Temperature profile 17 shows gas temperature rise through the catalyst unit as a portion of the fuel is combusted. After a delay, called the ignition delay time 16, the remaining fuel reacts to give the full temperature rise. In addition, the corresponding drop in the concentration of the fuel 18 along the same path is shown as a dotted line.
As shown in the bottom portion of FIG. 3, a portion of the fuel is combusted, without flame, in the catalyst resulting in an increase in temperature of the gas mixture. The mixture exiting the catalyst is at an elevated temperature and contains the remaining unburned fuel in air. This hot fuel and air mixture autoignites in a homogeneous combustion process in which the remaining fuel reacts in a radical reaction process to form the final reaction products of CO2 and H2O, and the temperature rises to the final combustion temperature for the total entering fuel and air mixture.
There is a similar problem with CO in the combustor output gases, in that regulations currently require less than about 100 ppm, and the movement is toward 10 ppm or less. A concern is that in reducing NOx levels, there may be a countervailing CO increase, such that in order to meet NOx limits, CO is exceeded. Thus, finding the window of low NOx and acceptable CO is increasingly difficult at the high FIG. 2, Region B, temperatures needed for efficient energy extraction.
Accordingly, for gas turbines that require combustor outlet temperatures in Region B in order to achieve the required drive-turbine design temperatures, and where emissions requirements demand NOx emissions levels below 3 ppm and CO on the order of 50-100 ppm or less, there is a need in the art for better control of the combustion process and ignition timing, and for improved combustion systems, apparatus and controls, in order to ensure that the NOx level produced in the combustion section of a gas turbine system can be maintained at lower levels, for example, 2 ppm or less while maintaining CO below about 10 ppm.
Summary, Including Objects and Advantages:
The invention comprises methods and apparatus, both devices and systems, for control of Zeldovich (thermal) pathway NOx production in catalytic combustion systems, and more particularly to control of NOx produced during combustion of liquid or gaseous fuels in the post catalytic sections of gas turbines by reducing combustion residence time in the HC zone through control of the HC wave, principally by adjusting the catalyst inlet temperature.
The invention arises out of the discovery that in the typical combustor having a physical or aerodynamic flame holder, the fuel and air mixture is combusted in a fixed position and does not move significantly as process conditions are varied. In contrast moreover, it has been discovered, unexpectedly, that in a catalytic combustor system, the location of the postcatalyst homogeneous combustion process that results in a temperature rise is not connected to the physical flame process or fixed flame holder, but rather is controlled by the catalyst exit gas conditions. Accordingly, the process of the invention comprises controlling the catalyst outlet temperature, which changes the HC wave location, which in turn controls the time period (residence time) during which the flame produces thermal NOx. Accordingly, the process of the invention comprises controlling the catalyst outlet temperature, which changes the HC wave location, which in turn controls the time period (residence time) during which the flame produces thermal NOx while maintaining a substantially constant adiabatic temperature in the post catalyst burn out zone. As soon as the gas mixture enters the drive turbine, work is extracted and the gas temperature drops significantly and NOx formation stops. Thus, in accord with the invention, by reducing the residence time at high post-catalyst reaction temperatures, NOx can be reduced to  less than 3 ppm, preferably  less than 2 ppm, while CO is maintained to within acceptable limits of  less than  50-100 ppm, and even to  less than 5-10 ppm.
This inventive feature is illustrated in FIG. 4, which shows a series of simple schematic drawings of a catalyst combustor system having a fuel injector, catalyst and post-catalyst homogeneous combustion zone feeding hot gas into a drive turbine. This series of figures illustrates schematically the change in the position of the homogeneous combustion wave, starting in FIG. 4A, with the HG wave being shown positioned downstream of the catalyst. The actual physical location of the HG wave is a function of the ignition delay time, tignition, as shown in FIG. 3, and the gas velocity. In FIG. 4B, the ignition delay is adjusted to be very long, so that after the ignition occurs and the high temperature is reached, the time that the gas mixture will be hot enough for thermal NOx formation is relatively short and NOx formation will be minimized. In FIG. 4A the ignition delay time is at an intermediate value and in FIG. 4C the ignition delay time is very short. In each of these later cases, the Zeldovich pathway NOx formed is progressively higher due to progressively longer times in which the gas mixture is at the high post-combustion temperature.
The catalyst outlet temperature can be changed by changing the operating conditions of the combustor system. For example, in a first embodiment of the control aspects of the invention, the amount of fuel fed to the preburner (shown in FIG. 1) is reduced, then the temperature entering the catalyst module will be lower and the temperature at the exit of the catalyst will also be lower. This lower temperature at the catalyst exit will move the homogeneous combustion wave farther downstream from the catalyst and closer to the turbine, thus reducing the level of thermal NOx formed. Similarly, increasing the fuel to the preburner will increase the catalyst outlet temperature, move the homogenous combustion wave upstream and increase the amount of thermal NOx formed. Other control embodiments are described below in the Detailed Description section of this Application.
The inventive control of the location of the HC Wave to reduce the thermal NOx output is an unexpected and very unusual aspect of catalytic combustion systems employing the partial downstream combustion technology described here.